The new 2015+ N/A V6's are 3.5 with 6-sp transmission (going to 3.3L in 2018, I believe, with 10-sp transmissions, which should improve the MPG even more). Looks like they do better than the old V6's, and better than the new 3.5L Tacoma's.
Yeah, the newer v6's do get slightly better mpg as you add more gears and fancy gadgets also help in that effort (cylinder deactivation, turbo's, ect.) but we're quickly approaching the point where there will be limited returns for updating those kinds of engines. I forgot where I saw the article about this topic, but it was an auto engineer who essentially wrote that there is limited performance and efficiency to wring out of these modern gasoline engines, and that if big gains are desired (which they likely will be due to the current regulatory environment) car-makers will have to switch to different power setups: hybrid, electric, CNG, turbo-diesel.
As for domestic V8's, both Ford and Ram V8's get much better MPG. By that site's own calculation, cost to run the Ford on an annual basis is 10% cheaper and 20% for the Ram. Having said that, I don't value those MPG figures, as it includes city miles. Too many variables there. Highway MPG is what I'd focus on. I have yet to see a Tundra get anything close to 20MPG. Yet domestics do it quite easily. My father's Tahoe gets 22MPG. That's a heavy beast, too (fully loaded LTZ). Plenty of folks with 5.0 Coyote and 5.7 Hemi get that as well. Part of confusion is that all of these manufacturers have same access to talent (engineers, designers, etc). Toyota is a big time player and could probably recruit any top level talent, but can't come up with something a bit more efficient?
Again, I'm not sure where you're getting that info, that or you're comparing apples to oranges. The 2016 V8's get 13/19 and 14/19 in the Ram and Ford respectively:
https://www.fueleconomy.gov/feg/Find.do?action=sbs&id=36810&id=36546&id=37052
That's not all that different from what the Tundra is rated at: 13/17. I've never heard of someone getting 22mpg with either of those domestic V8's; maybe if the driver was going 60mph, without stopping and no headwind...on a slight downhill. It might be possible, but it's definitely not normal.
The optional v6's are rated for better mpg because, well, they're v6's, so there is no point in comparing them to the Tundra's V8....though I highly doubt they maintain those kinds of mpg numbers in real-world driving.
I brought up Toyota's mpg, but the fact is it's not just Toyota...pretty much all BOF trucks have the same issue when it comes to gasoline engines and it's partly the reason why I'm glad to see more diesel options in upcoming models.